In general, an optical fiber cable comprises optical fibers disposed in an envelope. To ensure good signal transmission, it is generally desirable to keep the mechanical stresses to which the optical fibers are subjected while the cable is being handled down to a minimum, and in particular the stresses that occur during installation and use of the cable. That is why provision has been made over many years to associate optical fibers with a strength member of considerable size that provides decoupling between the structure of the cable, in particular the strength member, and the optical fibers, thereby ensuring that the mechanical forces exerted on the cable (in particular longitudinal forces in traction or compression) are withstood by the strength member.
Decoupling between the strength member and the optical fibers can be obtained during manufacture by providing for the optical fibers to be longer than the strength member, or by interposing layers having a low coefficient of friction and damping layers between the optical fibers and the strength member. Cables of those types are expensive to make and the resulting cable is bulky compared with the number of optical fibers it contains.
In order to mitigate those drawbacks, document FR-A-2 665 266 proposes making an optical fiber cable by providing longitudinal coupling between the optical fibers and the envelope in such a manner that mechanical forces, e.g. traction or compression forces, are shared between the envelope and the optical fibers. In the embodiment described in that document, the optical fibers are subdivided into modules, each of which is enveloped in a module sheath that is in contact with the optical fibers, and the various modules are grouped together in an envelope which is in contact with the module sheaths in order to provide longitudinal coupling therewith. The longitudinal coupling described in that document corresponds in practice to the optical fibers being in a disposition without extra length relative to the envelope, i.e. so that any lengthening or compression of the envelope gives rise to lengthening or compression of the optical fibers.
The force to be withstood by each optical fiber must remain below a threshold above which the transmission properties of the optical fiber are unacceptably degraded relative to the required performance. It will readily be understood that a cable made in that manner can withstand a larger force if it contains a larger number of optical fibers.
Also, when it is desired to manufacture cables of various dimensions, it is necessary not only to have corresponding tooling for making the envelopes, but also to stop the manufacturing line in order to change the tooling. For reasons of manufacturing costs, it is therefore preferable to minimize the number of cables having various dimensions. Also, the cost of optical fibers constitutes a very large fraction of the cost of a cable, so it is not desirable for a cable to have a large number of optical fibers solely for the purpose of distributing forces between them.